In cases you haven't installed all toolboxes via the installer provided on the download page you can later install them via the plugin manager. This page describes the necessary steps.
Let's assume that you have installed the Sentinel Toolboxes and decide later that you also want to work with SMOS data. Hence, you want to install the SMOS-Toolbox.
Start SNAP Desktop.Open the Plugin Manager. From the menu select Tools --> Plugins
Select the toolbox you would like to install. Here the SMOS-Box Kit Module.If you like you can also select more than one toolbox.
A Kit module combines all necessary modules needed for the toolbox. Click on Install in order to start the installation process.
If the toolbox needs updated versions of modules which are already installed you are notified about these updates.
Click on Next
It might be necessary to accept the license agreement.Click on Install. The download and the installation starts.
In order to finalise the installation SNAP Desktop needs to be restarted.Choose if you want to restart now or later and click finish.
When starting SNAP Desktop the next time the new functionality will be available